text: import matplotlib
  import matplotlib.pyplot as plt
  import numpy as np
  from helpers import assert_equality
  
  
  def plot():
      fig = plt.figure()
      x, y = np.ogrid[-10:10:100j, -10:10:100j]
      extent = (x.min(), x.max(), y.min(), y.max())
      cmap = matplotlib.cm.get_cmap("gray")
      plt.imshow(x * y, extent=extent, cmap=cmap)
      plt.colorbar()
      return fig
  
  
  def test():
      assert_equality(plot, "test_heat_reference.tex")
      return
  